Stabilized reducing agents



United States Patent Delaware No Drawing. Filed Nov. 30, 1962, Ser. No.243,178 5 Claims. (Cl. 260-448) This invention concerns a novelpreparation of a stable aluminum hydride complex.

More particularly this invention relates to a new and improved method ofpreparing stable aluminum hydride alkyl etherate complexes in good yieldfrom readily available starting materials.

Aluminum hydride is a Well known and useful reagent for reducingunsaturated organic compounds as well as reducible groups of organiccompounds such as carbonyl, hydroxy, nitro and related groups.Unfortunately the presently available forms of the free aluminum hydrideare difiicult to handle, ship or store because of their extremereactivity with many substances. For example,

lithium aluminum hydride reacts violently and sometimes explosively withmoisture and certain oxygen containing agents such as nitromethane andmethanol and it is inactivated by contact with oxygen and CO To moderateits reactivity and decrease the risks in handling it, the hydride can bemade available in the solid state in the form of its ethyl etherate.This complex retains the reducing propensities of lithium aluminumhydride yet unlike this hydride does not react violently with oxygen oroxidizing agents. Therefore, an improved method of preparing aluminumalkyl etherates particularly the aluminum hydride ethyl etherates is avaluable adjuvant to synthetic organic chemistry and is much to bedesired.

The present method of preparing aluminum hydride ethyl etherate (Finholtet a1., J.A.C.S. 69, 1119' (1947)) is to react diethyl ether solutionsof lithium aluminum hydride and anhydrous aluminum chloride to form thedesired etherate product and lithium chloride as a byproduct. The priorart process is disadvantageous in several respects. For one, lithiumchloride is almost always a contaminant of the etherate product. Sincelithium chloride is somewhat hygroscopic, it imparts this undesirablevcharacteristic to the water sensitive product. Another disadvantage ofthe process is that unless the separation of the by-product lithiumchloride is made rapidly, a secondary and competing reaction produces apolymeric aluminum hydride which precipitates out. This polymer is ofless value as a reducing agent and its formation is undesirable. Theformation of the polymeric aluminum hydride can be prevented by theaddition of aluminum chloride but the resulting complex introduces theundesired chloride ion as an impurity. Furthermore, the etheratesproduced by this process vary in content from batch to batch and areinconstant in their stoichiometric proportions.

Thus it is an object of this invention to prepare aluminum hydride alkyletherates uncontaminated with lithium chloride in good yield fromreadily available. starting materials and in constant stoichiometricproportion.

It is a further object of this invention to prepare aluminum hydridealkyl etherates particularly aluminum hydride ethyl etherates withoutthe contemporaneous preparation of polymeric aluminum hydrides.

Yet another object of this invention is to produce a convenient sourceof anhydrous aluminum perchlorate without resorting to the use of thehazardous starting material anhydrous perchloric acid.

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The aforementioned objects are achieved by the novel process of thisinvention which is set forth below:

wherein R is an alkyl radical which may be the same or different andcontains 1-8 carbon atoms.

In practice the anhydrous silver perchlorate reactant is dissolved inthe appropriate dialkyl ether solvent and a solution of anhydrousaluminum chloride in the anhydrous ether is added to the stirredsolution. The precipitated silver chloride is filtered off leaving thedesired anhydrous aluminum perchlorate in the filtrate. The aluminumperchlorate-dialkyl ether solution is chilled down to 78 to 0 C. andblanketed with an inert gas such as nitrogen or argon. At this point asolution of the lithium aluminum hydride in the dialkyl ether is addedto the chilled stirred solution dropwise. The chilled mixture is allowedto stir for an additional 5-60 minutes at the chilled temperature andthen the reaction solution is allowed to Warm up to approximately roomtemperature and the stirring continued for atleast 10 minutes preferablyfrom 30 minutes to 60 minutes. During the above addition the aluminumhydride alkyl etherate product is precipitated. After the last stirringstep the product is filtered and washed with the dialkyl etherused asthe solvent. The precipitated product is sufiiciently heat stable to bedried in vacuo at 20 to 80 C. preferably at about 35 C. Drying time isdependent upon the temperature used and the strength of the vacuum.

As indicated by the above brief description of the process the processconditions are relatively flexible. For example the preparation of thealuminum perchlorate solution (Equation 1) can be accomplished at low tohigh temperatures or at ambient temperatures. Little or no advantageaccrues in-using extremes of temperature so for convenience sake thereaction is run at substantially ambient temperatures. However, it isdesirable to remove the precipitated silver chloride and utilize thealuminum perchlorate solution as soon as possible because of itstendency to pick up moisture from the atmosphere. Similarly aside fromrequiring low reaction temperature and an inert atmosphere no particularexperimental conditions are critical to the success of the secondreaction. Neither reaction requires any special order of adding thereactants, or any particular operating pressure. For example the orderof adding the reactants can be reversed and sub through superatmospheric pressures can be used. In both reactions stoichiometricquantities of the reagents is desirable but not necessary. As indicatedby the aforementioned reactions the dialkyl ether used can range from 18carbon atoms in length, and the alkyl groups can be the same or they candiffer.

The novel process of this invention is advantageous in several respects.For example yields of over are obtained and the alkyl etherate complexis substantially free from contaminating lithium chloride. In additionthe resultant product is relatively stable to heat, moisture and oxygenand the production of the non-stoichiometric, less valuable polymericaluminum hydride is avoided.

The reactants, aluminum chloride, silver perchlorate, lithium aluminumhydride and the dialkyl ethers are all articles of commerce and theirpreparation is well documented in the chemical literature includingChemical Abstracts. In each instance chemically pure anhydrous Example1.Preparatin of aluminum hydride ethyl etherate (A) Aluminum PerchlorateS0luti0n.A 620 part by weight portion of anhydrous silver perchlorate isdissolved in 1050 parts by weight of anhydrous diethyl ether. Theresultant solution is added to a solution of 135 parts by weight ofanhydrous aluminum chloride in 1050 parts by weight of anhydrous diethylether contained in an appropriate reaction vessel fitted for eflicientcooling and stirring. The white precipitate of silver chloride isquickly filtered oil and washed with additional anhydrous diethyl ether.The ether washings are added to the aluminum perchlorate solution inether. Analysis confirmed the precipitate to be silver chloride and thepresence of aluminum perchlorate in the diethyl ether filtrate. Basedupon the aluminum chloride starting material a 98% yield (430 parts byweight of aluminum perchlorate product) is present.

(B) Aluminum Hydride Ethyl Etherale.The aluminum perchlorate diethylether solution is cooled to 78 C. and blanketed with dry nitrogen. Asolution of 300 parts by weight of anhydrous lithium aluminum hydride in1050 parts by weight of diethyl ether is added dropwise to the stirredsolution of aluminum perchlorate diethyl ether solution. A whiteprecipitate is formed during the addition of the lithium aluminumhydride solution. After the addition is complete the stirring iscontinued for 15 minutes at 78 C. and then for 30 minutes at room'temperature. The precipitate is filtered ofi and washed with anhydrousether and dried in vacuo at 65 C. for 16 hours. A 96.3% yield of productis obtained (228 parts by weight) as a white powdery solid. Hydrolysisand infra-red analysis confirmed the identity of the product as aluminumhydride ethyl et-herate and the formula as' [(A1H (C H O]. Analyticaltests for lithium and perchlorate ions are negative.

Example 2.-Preparati0n of other aluminum hydride alkyl etherates Usingthe following dialkyl ethers as solvents and the procedure disclosed inExample 1 the representative compounds shown in the left hand column areprepared in essentially quantitative yields.

Ether: Product Di-n-propyl ether Aluminum hydride n-propyl -etherate.Diisopropyl ether Aluminum hydride-isopropyl etherate. Dibutyl etherAluminum hydride-butyl etherate.

In each instance the identity of the product is estab wherein R is analkyl group having from 18 carbon atoms, removing the precipitatedsilver chloride formed from said dialkyl ether solution of aluminumperchlorate, cooling said dialkyl ether solution of aluminum perchloratebelow 0 C. and contacting said cooled solution with a solution oflithium aluminum hydride in the aforementioned dialkyl ether in an inertatmosphere, and recovering the precipitated aluminum hydride dialkyletherate product therein.

2. The process of claim 1 wherein the dialkyl ether is diethyl ether.

3. The process of claim 1 wherein the dialkyl ether is di-n-propylether.

4. The process of claim l'wherein the dialkyl ether is diisopropylether.

5. The process of claim 1 wherein the dialkyl ether is dibutyl ether.

No references cited.

BENJAMIN HENKIN, Primary Examiner. CARL D. QUARFORTH, Examiner.

1. A PROCESS FOR PREPARING ALUMINUM HYDRIDE ALKYL ETHERATES COMPRISINGTHE STEPS OF CONTACTING DIALKYL ETHER SOLUTIONS OF ALUMINUM CHLORIDE ANDSILVER PERCHLORATE, SAID DIALKYL ETHERS BEING OF THE FORMULA: